There continues to be considerable interest in producing optical amplifiers for amplifying weak optical signals in both local and trunk optical networks. The high data rates and low optical attenuation associated with fiber optic lengths are well-established and continue to become more appreciated as fiber lengths become more economical compared with electrical coaxial cable alternatives. In spite of the relatively low magnitude of optical signal loss during transmission, the intrinsic linear attenuation law of lightwave energy in optical fibers necessitates optical repeater nodes to amplify and/or regenerate the digital optical bitstreams or analog signals in long-haul terrestrial and undersea communication systems. Typically, unrepeated distances extend from 30 to 70 kilometers in length, depending upon the fiber loss at the selected transmission wavelength, which is ordinarily 1.31 or 1.55 microns, respectively.
One non-invasive approach to amplifying an optical signal in a fiber optic is presented in U.S. Pat. Nos. 4,955,025 and 5,005,175 entitled, "Fiber-Optic Lasers and Amplifiers" and "Erbium-Doped Fiber Amplifier," respectively. In these patents, a doped optical fiber is transversely coupled to a pump so that a weak optical input signal at a specific wavelength within the rare-earth gain profile experiences a desired amplification. Pumping is effected by a separate laser or lamp which emits photons of appropriate energy, i.e., higher than that of the signal wavelength. Electrons in the doped fiber are excited from the ground state to one or more pump bands. The electrons then decay an amount corresponding to the wavelength at which the device operates. When a photon at the laser wavelength interacts with an excited atom, stimulated emission occurs. An output photon can thus originate from either previous spontaneous emission, stimulated emission, or an input signal.
Since erbium-doped amplifiers only operate at a specific wavelength, i.e., 1.53 .mu.m-1.55 .mu.m, other approaches to non-invasive optical amplifiers, operable for example at 1310 nm, are under investigation using semiconductor materials and variations of the rare-earth doped fibers. To date, however, serious problems have plagued development of these devices. Namely, semiconductor amplifiers have been unable to provide sufficient gain, without still having significant noise problems. Rare-earth doped fiber amplifiers have also suffered from a variety of problems.
For example, neodymium (Nd) doped fibers have problems in obtaining sufficient gain at 1310 nm while maintaining the high quality, low-loss characteristics of the optical fiber. Specifically, excited state absorption in silica-based, Nd-doped fiber push the spectral range of any reasonable gain out to beyond 1330 nm resulting in substantial reduction in the usable spectrum. Further, using non-silica fibers, such as fluorozirconate or phosphate glass fibers, poses significant fabrication problems and raises questions of environmental stability. The alternative to neodymium in the 1300 nm window in optical fiber is Praseodymium (Pr), which also has had its share of difficulties. As with Nd-doped silica fiber, Pr-doped fiber requires a non-silica fiber host to achieve sufficient gain to be usable. In fact, to date only fluorozirconate (ZBLAN) fiber has proven capable of generating sufficient gain. Unfortunately, Pr-doped fibers, ZBLAN or otherwise, also require pumping at a wavelength of 1020 nm, which is not a standard laser transition. Thus, there have been significant problems generating the needed power levels at these odd wavelengths.
In view of the above, there remains a need in the optical communications art for an improved optical amplifier and amplification approach providing amplification characteristics commensurate with those attained by erbium-doped fibers at 1550 nm, but operable at any optical wavelength employed within an optical fiber, such as 1310 nm. The present invention provides an optical amplifier architecture and amplification process which addresses this need.